1. Field of the Invention
This invention relates to methods and apparatus for preheating particulate feed material for a rotary kiln with heated exit gases from the kiln, as is utilized, for example, to produce Portland cement. In particular, this invention relates to such preheaters that comprise an assembly of cyclone dust separators and this invention may be applied to such a preheater having one or more auxiliary burners to additionally heat feed material after the feed material has been preheated but prior to the feed material entering the rotary kiln.
2. Description of the Prior Art
Preheaters for rotary kilns known to the prior art that involve preheating finely divided raw materials suspended in and moving generally counter to the flow of heated kiln exit gases flowing through one or more cyclone dust separators are disclosed in such as Czechoslovakian Pat. No. 48,169 of 1934; published German patent application K 156,877 of 1940; U.S. Pat. Nos. 2,648,532 and 2,663,560 of 1953; and many others such as are identified in U.S. Pat. No. 3,441,258 of 1969.
Preheaters of the aforementioned type but which are additionally provided with one or more auxiliary burners (projecting into a cyclone dust separator or an auxiliary furnace) to additionally heat feed material after the feed material has been preheated but prior to the feed material entering the rotary kiln are disclosed in patents such as U.S. Pat. Nos.; 3,235,239 of 1966; 3,452,968 of 1969; 3,507,482 of 1970; 3,752,455 of 1973; 3,834,860 and 3,843,314 of 1974; 3,869,248, 3,873,331, 3,881,862, 3,891,382, 3,891,383, 3,904,353, 3,910,754, 3,914,098, 3,925,091 and 3,926,651, all of 1975; and 3,932,116 and 3,932,117 of 1976.
It is known from the disclosure in the aforementioned U.S. Pat. No. 3,834,860 for a system developed by Onoda Cement Co., Ltd. of Japan, that the heat efficiency of preheaters of the aforementioned types can be improved by recovering the hot waste air from the cooler of a cement plant, and that the stability of the preheater with an auxiliary furnace using such air to support combustion of fuel, improves with the increase of the temperature of the said hot waste air. However, the hot waste air from the cooler cannot be used efficiently in all cases because the length of the hot air duct from the cooler to the auxiliary furnace sometimes must be of a length almots equivalent to that of the kiln because in the design of the plant it is inevitably necessary to install the auxiliary furnace at a considerable distance from the cooler. In addition, the high abrasion effect of the fine suspended particles of clinker contained in the waste air from the cooler calls for special abrasion preventive measures and makes it impractical to install in the duct a blower which is highly vulnerable to the said abrasion effect. It is further known from the prior art and disclosed for example in U.S. Pat. No. 3,869,248 that a throttle resistance may be provided within the flue connected to the outlet of the kiln gas to make use of the suction force of a blower taking suction from a first stage cyclone separator to draw hot air from the cooler into the preheater. However, the installation of the throttle resistance has the disadvantage of increasing both the air flow resistance and the power consumption of the suction blower.
The waste air from the cooler can be efficiently used for drying the raw materials depending upon the design of the cement manufacturing device, but when that is done, waste air from the cooler is not available to be supplied to the auxiliary furnace of the preheater. Further, in other special cases where air cooling of clinker is not the way clinker is cooled, no hot waste gas from the cooler is available for use either in the kiln or in the auxiliary furnace of the preheater.
The waste gas discharged to the atmosphere from a preheater used for cement manufacturing usually has a temperature of approximately 350.degree. C., which is ascribable to the fact that the sensible heat of the waste gas brought from the pre-sintering range to the preheating range is far higher than is required for preheating the raw materials to the pre-sintering temperature. Unless the volume of such waste combustion gas is decreased, therefore, increase of the preheating ranges does not result in the decline of the gas temperature at the preheater outlet to the neighborhood of the inlet material temperature and such gas temperature above material inlet temperature represents wasted heat energy.